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MASTER GARDENERS: Why do leaves turn yellow?

By Bill HayesColumnist

Oct 28 2012 9:02 am Oct 28 9:05 am

The Clemson Extension office receives all types of calls about problems in the garden. One of the most common questions is ďWhy are my leaves turning yellow?Ē The question may refer to plant, tree or vegetable leaves, and the answer may be different for each type of plant. Letís look at a few of the possibilities with the understanding that there could be more reasons than we can cover in our allowed space.

House plants

Too much water is the problem in the majority of cases when a plantís health is declining. Several factors determine the appropriate frequency of watering: type of plant, temperature/humidity, pot size, light and drainage. Checking the plant on a regular basis instead of adhering to a strict schedule is better than overwatering, which drowns the roots. Some plants are heavier feeders than others and will need to be fed more often. Using a weakened solution of a soluble fertilizer when watering your plants is better than a strong dose every once in a while. Aphids, scale insects, mites and root damage due to fungal infections can cause yellowing of the leaves on houseplants. Mite infestation is facilitated by warm, dry environmental conditions. While most plants respond well to bright light, individual plants have different needs both for the amount and the intensity of the light they require to grow and thrive. Yellow leaves can indicate that your plant is either receiving too much or too little light. Investigate the proper light requirements for your specific plant. Yellow leaves, especially if they are on the lower portions of the plant, can be an indication that the roots are overgrown in the pot. Rubber plants are particularly prone to this problem. Some loss of foliage is normal. The length of time the leaves remain on the plant depends on the species.

Outdoor plants

There are many reasons why an outdoor plantís leaves may turn yellow and possibly die. Sometimes this yellowing process is perfectly natural, and other times it is caused by environmental changes or pests. Every year in the fall, deciduous perennial plant species enter a state of dormancy, where they shut down their metabolism for the winter. As winter nears, plants and trees absorb the nutrients in their leaves, then shed the leaves to the ground below. With the coming of spring, deciduous plant come out of dormancy and grow new leaves for the coming year. Leaves may turn yellow on plants if they do not receive enough water.

Dropping leaves is a defensive mechanism by the plant to conserve water because plants easily lose water through their leaves. If a plant remains without water long enough, all leaves will turn yellow and die, as will the entire plant. Our current drought conditions are certainly the cause of decline for many of our plants and trees. Environmental variables like heat spells, frosts, drying winds and sun burn can cause outdoor plants to become stressed, causing the leaves to yellow and drop. Once the environment becomes stable and the plant becomes acclimated, most plants should recover.

Disease or pest infestation may also cause a plantís leaves to turn yellow. Common pests notorious for this are spider mites, lace bugs and aphids. These small insects suck nutrients from leaves, stems and roots of a plant. Fungal infestations like root rot and virticillium wilt will also cause discoloration and dropping of leaves. Nematodes are microscopic worm-like creatures that attach themselves to roots and suck out moisture. A special soil test is usually required to determine if they are the problem for a dying plant. Nutrient deficiency will also cause yellowing of the leaves. This happens when the plant draws the nutrients the soil lacks from its own leaves. The most common deficiency is Nitrogen deficiency, which causes all leaves to turn light yellow and growth to be stunted. Iron deficiency causes yellowing between leaf veins, and potassium deficiency causes leaf tips to yellow and die-back.

Our current weather conditions are the major cause of plant and tree problems. Our sandy soil conditions add to the problem by not holding water long enough to satisfy the plantís requirements. Normal hand watering from a garden hose simply isnít enough. This type of irrigation usually dries before reaching the root system and does little to help the plant. Soaker hoses provide moisture where it is needed and can be left on for extended periods of time. Shallow rooted trees such as dogwood, Bradford pear, and river birch along with plants like azalea and boxwood require lots of water during hot and dry weather or they will show signs of wilting.

Earlier we mentioned that too much water can cause leaves to turn yellow. While almost all plants require a sufficient water supply to thrive, too much can kill the plant. The root system absorbs water along with oxygen to support plant growth and nutrition. If too much water accumulates in the root system, oxygen is cut off and the plant will begin to fail. Once the water level is normal the plant may recover and eventually get back to normal.

Leaf yellowing is a symptom of something going wrong and can be caused by numerous things. A little detective work may be needed to determine the real cause.

Trees

Why do leaves change color? Itís one of those questions Ė like, why is the sky blue? Most simply, to survive the winter, deciduous trees need to store nutrients in their roots, which means they must absorb the nutrients in their leaves. Changes in color are triggered as the trees absorb essential nutrients. Hereís how it works: Throughout the warm sunny months, trees are lush and green because theyíre working hard. Tree leaves are green because the abundance of the pigment chlorophyll, which is essential to converting sunlight, water and carbon dioxide into energy-rich sugars. If plants hadnít figured out the trick of photosynthesis, weíd all be out of luck, since the energy humans need to live comes from plants, or the animals that eat plants. Tree leaves are also busy using other essential nutrients, like nitrogen and phosphorus (the same main ingredients in most store-bought fertilizers, or in compost), so these nutrients are abundant in summer foliage. As summer wanes, changes in tree leaves are triggered by the cooler temperatures, changes in rainfall and weather, and most of all, the shortening of daylight hours. Much of the change happens without our knowing it, as trees begin to absorb essential nutrients and store them in their roots so they are available for the following spring. As the trees absorb the last of the chlorophyll, however, the brilliant colors we associate with autumn begin to appear. Eventually, the leaves lose their ability to stick to the branches, they wither, turn brown and fall to the earth. Another cycle is fulfilled and we wait for the warm temperatures of spring to start again.

Bill Hayes has been in Aiken since 1982 after moving from Chicago, Ill. He was in the chemical process industry for more than 40 years before retiring in 1999.

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